In a semiconductor laser, it is important to decrease light absorption in an active layer in the vicinity of mirrors, thereby suppressing local heating in the areas, so that a light output of the semiconductor laser becomes high. For this purpose, an active layer is removed at the vicinity of facets for emitting light to provide windows which are transparent at a lasing wavelength, so that the optical absorption is avoided at the facets.
One type of the windows are explained in a report entitled "Large optical cavity AlGaAs buried heterostructure window lasers" by H. Blauvelt, et al which is described on pages 1029 to 1031 in "Appl. Phys. Lett. 40 (12), 15 June 1982", wherein the windows are provided in a structure to be buried the facets of active section by a crystal having a mixed crystal ratio of Al larger than that of an active layer.
This type of a semiconductor laser will be fabricated as follows.
At first, an n-Al.sub.x Ga.sub.1-x As cladding layer, and an active layer are successively grown on an n-GaAs substrate .noteq. by liquid phase epitary, and the cladding layer and the active layer are partially removed in wet etching at both ends by 25 .mu.m. The etched sections are buried with p-Al.sub.z Ga.sub.1-z As layers and n-Al.sub.z Ga.sub.1-z As layers each having a mixed crystal ratio of Al smaller than that of the cladding layer and larger than an optical guiding layer (to be grown next) to provide high resistive optical window section. The optical guiding layer of p-Al.sub.y Ga.sub.1-y As, a cladding layer of p-Al.sub.x' Ga.sub.1-x' As, and an n-GaAs layer are then grown on the active layer and the optical window section. Thereafter, Zn is selectively diffused into the n-GaAs layer and an upper portion of the cladding layer.
Then, an insulation film is formed on the n-GaAs layer in the vicinity of the facets to be cleaved in the center section thereof, and n- and p- electrodes are provided on the bottom surface of the n-GaAs substrate and the upper surface of the n-GaAs layer, respectively, to complete the window semiconductor laser.
In this window semiconductor laser, a maximum gain wavelength (light emitting wavelength) is obtained as a result of re-combination of carriers injected in a large amount into the center section, while no absorption of light occurs in the window section in the vicinity of the facets having a bandgap energy larger than the center section, so that the optical radiation power is increased. In fact, a catastrophic optical damage power level is increased from several tens mW to 150 mW by the provision of the window regions.
However, this window semiconductor laser has disadvantages in that light propagating through the optical guiding layer reflects and scatters at coupling portions between the window regions including the p- and n-Al.sub.z Ga.sub.1-z As layers and the active layer to result in the increase of coupling losses and a threshold current, the decrease of an efficiency, and the occurence of thermal saturation, so that a high optical radiation power is difficult to be obtained, when the p- and n-Al.sub.z Ga.sub.1-z As layers do not coincide in thickness with the active layer. For the purpose of overcoming these disadvantages, the p- and n-Al.sub.z Ga.sub.1-z As layers are necessary to be controlled in thickness. However, this thickness control is difficult, although the structure of this window semiconductor laser is well adapted to liquid phase epitaxy, wherein a speed of growing Al.sub.z Ga.sub.1-z As on Al.sub.z Ga.sub.1-2 As is much lower than that of growing Al.sub.z Ga.sub.1-z As on GaAs. For these reasons, this window semiconductor laser is difficult to be fabricated by use of this growth technology.
This window semiconductor laser has a further disadvantage in that a focused beam diameter of an emitted light is difficult to be an order of microns due to the fact that light which is controlled in mode in the active layer is expanded transversely in the window sections, because the window sections do not have a function of controlling a transverse mode. For a light source in an information processing system, especially, it is very important that a light beam is well controlled in a horizontal, transverse mode to increase reliability of the system and an effective availability of the light beam. A structure of controlling a horizontal, transverse mode can be added to this window semiconductor laser. As a result, however, the fabrication process thereof becomes very complicated.